major: Switch to forward slashes(+more) to fix compilation on Linux (#1403)

Notably also adds some metadata files for NixOS 

* add support for linux clang cross compiles

* add linux clang instructions

* un-capitalize Mob.horse.*

* update the description in flake.nix

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Co-authored-by: Loki <lokirautio@gmail.com>

Minecraft.Client/Durango/Network/DQRNetworkManager.cpp

@@ -5,11 +5,11 @@
 #include <collection.h>
 #include <ppltasks.h>
 #include <ws2tcpip.h>
-#include "..\Minecraft.World\StringHelpers.h"
+#include "../Minecraft.World/StringHelpers.h"
 #include "base64.h"
 
 #ifdef _DURANGO
-#include "..\Minecraft.World\DurangoStats.h"
+#include "../Minecraft.World/DurangoStats.h"
 #endif
 
 #include "ChatIntegrationLayer.h"